A method for operating an internal combustion engine is described in German Patent Application No. DE 198 13 381, in which fuel is injected either in a first operating mode or in a second operating mode. In the first operating mode, a so-called stratified operation is provided, and in the second operating mode, a so-called homogeneous operation is provided. Stratified operation is used especially for small loads, whereas homogeneous operation comes into use for greater loads. During the stratified operation, which corresponds to a comparatively fuel-efficient operation of the internal combustion engine, there occurs an increased NOx emission of the internal combustion engine.
In stratified operation, a throttle valve in the intake region of the internal combustion engine is largely opened and the combustion is established essentially only by the injected fuel mass. Homogeneous operation approximately corresponds to the operating mode of internal combustion engines in which, in the usual method, fuel is injected into the intake region of the internal combustion engine. In homogeneous operation, the throttle valve is opened and closed as a function of the required torque, and the fuel quantity to be injected is determined as a function of the aspirated air mass.
The increased NOx emissions occurring in the first operating mode are no longer able to be made completely harmless by using a three-way catalytic converter.
Instead of this, NOx storage catalysts are used, which store the NOx being created in the stratified operation, from time to time.
German Patent Application No. DE 197 39 848 describes various operating methods for an internal combustion engine, in whose exhaust region an NOx adsorption catalyst is situated. Because of the finite storage capacity of the NOx adsorption catalyst, a regeneration of the NOx adsorption catalyst must be provided in between, which takes place because of a supply of hydrocarbons and/or carbon monoxide which are made available within the engine. Making available hydrocarbons/carbon monoxide comes about because, in the regeneration phase, the internal combustion engine is operated in a rich mode, the air ratio lambda being less than, or at most equal to 1.
Several possibilities for detecting the end of the NOx storing phase are described. One possibility provides a calculation of the NOx mass that is stored in the NOx adsorption catalyst, and this is calculated with the aid of a model of the NOx adsorption catalysts and known internal combustion engine operating characteristics variables.
Several possibilities for detecting the end of the regeneration phase are also described. One possibility provides the use of a lambda sensor situated downstream from the NOx adsorption catalyst, which detects a rich breakthrough that is occurring towards the end of the regeneration phase.
Such a possibility of detecting the end of the regeneration phase using a lambda sensor situated downstream from the NOx adsorption catalyst is furthermore described in German Patent No. DE 197 55 600. As soon as the lambda sensor detects a transition from a lean to a rich exhaust gas, this means that the NOx adsorption catalysts no longer makes available enough oxygen for the oxidation of the hydrocarbons or of the carbon monoxide, and that the NOx storage device has been largely regenerated.
The detection of the end of the regeneration presupposes a rich breakthrough, which is minimized by reducing the quantity of the reduction means respectively supplied in a time interval, with respect to that quantity, in response to whose supply in a preceding supply phase a reaction of the lambda sensor occurred.
In the reference book “Otto-Motor-Management/BOSCH”, Vieweg Verlag (Publishing House), 1st edition, 1998, pages 22-23, a broadband lambda sensor is described, having a sensor chamber which is connected to a gas compartment, in which the gas to be examined is located, via a diffusion barrier. Situated in the sensor chamber is an inner pump electrode which, together with an outer pump electrode and an electrolyte that is situated between the pump electrodes and conducts oxygen ions, forms a pump cell. Using the pump cell, oxygen ions of the gas can be pumped through the electrolyte out of, or into the sensor chamber. In addition to the pump cell, a measuring cell is provided that is disposed between the inner pump electrode and a reference-gas electrode, an electrolyte conducting oxygen ions likewise being situated between the inner pump electrode and the reference-gas electrode. The measuring cell corresponds to a Nernst cell, in which the potential difference forming in the thermodynamic equilibrium between the inner pump electrode and the air reference electrode is proportional to the logarithm of the ratio of the oxygen partial pressure of the gas in the sensor chamber and the oxygen partial pressure of the gas in the air reference.
The present invention is based on an object of providing a method for operating an internal combustion engine in whose exhaust region, in the direction of flow of the exhaust gas, a first broadband lambda sensor, an NOx adsorption catalyst and a second broadband lambda sensor are situated, in which the NOx adsorption catalyst cyclically stores the NOx emissions of the internal combustion engine and, within the scope of a regeneration, by the operation of the internal combustion engine using a rich, or at least a stoichiometric air fuel mixture, is regenerated, and of providing a device, for carrying out the method, which ensure great accuracy in the processing of the signal made available by the first broadband lambda sensor.